Problem: A circle with circumference $6\pi$ has an arc with a $\dfrac{6}{5}\pi$ radians central angle. What is the length of the arc? ${6\pi}$ ${\dfrac{6}{5}\pi}$ $\color{#DF0030}{\dfrac{18}{5}\pi}$
Solution: The ratio between the arc's central angle $\theta$ and $2 \pi$ radians is equal to the the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{2 \pi} = \dfrac{s}{c}$ $\dfrac{6}{5}\pi \div 2 \pi = \dfrac{s}{6\pi}$ $\dfrac{3}{5} = \dfrac{s}{6\pi}$ $\dfrac{3}{5} \times 6\pi = s$ $\dfrac{18}{5}\pi = s$